Inkjet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The frequency and amplitude of the firing signals correspond to the selective activation of the printhead actuators. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals.
Throughout the life cycle of these inkjet imaging devices, the image generating ability of the device requires evaluation and, if the images contain detectable errors, correction. Missing inkjets or weak inkjets are an error condition that affects ink image quality. A missing inkjet is an inkjet that does not eject an ink drop in response to a firing signal. A weak inkjet is an inkjet that responds intermittently to a firing signal or that responds by ejecting ink drops having a mass that is less than the ink drop mass corresponding to the characteristics of the firing signal for the inkjet. Systems and methods have been developed that compensate for missing or weak inkjets, but the missing or weak inkjets must be detected before these systems and methods can be activated.
Before an inkjet imaging device leaves a manufacturing facility, the device is typically tested to determine whether the printhead has a number of missing or weak jets that would adversely affect image quality. Additionally, inkjets in the printheads of an inkjet imaging device may begin to exhibit missing or weak inkjet characteristics. These changes arise because the device and its environment may experience temperature instabilities, air bubbles, dust, or other debris, which may cause components of the device to shift or operate unreliably. These conditions may cause the intrinsic performance of the device to change reversibly or irreversibly. Consequently, the inkjets of the printheads in an inkjet imaging device require evaluation at various intervals during the operational life of the device to detect changes in the performance of the inkjets. Sometimes these evaluations and adjustments are made at time or usage intervals, while at other times the adjustments are made during service calls made by trained technicians.
Detection of missing and weak inkjets in known imaging systems requires the printing of a test pattern. The printed test pattern is then scanned by an image scanner to generate image data of the pattern. These image data are then analyzed with reference to test pattern data used to print the pattern to detect areas where ink is expected, but is not detected in the image data of the pattern. While some of these systems are effective in identifying missing inkjets, they require a test pattern to be printed for analysis. The printing of the test pattern interrupts the availability of the system for producing images for the owner of the system. In web printing systems, the test pattern may be printed between pages of a print job, but the test pattern must be removed from the web so the customer can use the documents printed on the web. Being able to detect missing inkjets from customer generated images without requiring knowledge of the image content would be useful.